Conventionally, various types of wire-dot print heads for use in a wire-dot printer have been known. Spring-charged wire-dot print heads are widely used in applications where high speeds and high printing forces are desired. FIG. 1 is a sectional view of a wire-dot print head of the spring charge type.
In the figure, print wires 1 extend generally parallel with each other. The front or forward (upper as seen in the figure) parts of the print wires 1 extend through an aperture 8a provided in the front end of a nose 8b forming the front part of a wire guide 8. When the print wires 1 are driven forward, in a manner later described, their front ends strike a print medium, such as print paper PP on platen PL via an ink ribbon IR thereby performing wire-dot printing.
Rear (lower as seen in the figure) ends of the print wires 1 are fixed to inner or first ends of armatures 2 which are disposed to extend radially. A plate spring 3 comprises radial parts 3a which are fixed to rear surfaces of outer or second ends of the armatures 2. The plate spring 3 also comprises an annular part 3b which is integrally connected the outer ends of the radial parts 3a and is clamped between the front end of a cylindrical permanent magnet 4 and the rear surface of an annular part 5b of a front yoke 5.
The front yoke 5 also comprises radial parts 5a having outer ends integrally connected by the annular part 5b. Each armature 2 is positioned between adjacent radial parts 5a of the front yoke 5, with a slight gap on each side. Cores 6 extend from a disk-shaped base yoke 10 forward and their front ends are facing the lower surfaces of the armatures 2. Coils 7 are wound oil the cores 6 to form electromagnets EM for the respective armatures 2 and hence for the respective print wires 1. The rear end of the cylindrical permanent magnet 4 is connected to the periphery of the disk-shaped base yoke 10. The permanent magnet 4, the annular part 3b of the plate spring 3, the annular part 5b of the front yoke 5 and an annular part 8c of the wire guide 8 form a cylindrical wall of the print head.
The magnetic flux from the permanent magnet 4 passes through the annular part 5b of the front yoke 5, the radial parts 5a of the front yoke 5, the armatures 2, the cores 6 and the base yoke 10, thereby attracting the armatures 2 toward the cores 6, bending the radial parts 3a of the plate spring 3. When the electromagnets EM are energized to generate a magnetic flux canceling the magnetic flux from the permanent magnet 4, the armatures 2 are released and the print wires 1 are driven forward by virtue of the recovery force of the plate spring 3.
Further details of this action are described in, for example, Japanese Patent Kokai Publication No. 53860/1989. The period for which the electromagnets are energized is determined by a drive time signal DT.sub.1 shown in FIG. 2. Another drive time signal DT.sub.2 is used to provide, subsequent to the energization time, a period PR1 in which currents due to the electromotive forces induced in the electromagnets are allowed to flow through a certain current path. Subsequent to the first period, currents which are also due to electromotive forces, flow through another current path for a certain period, denoted PR2, until the current falls to zero.
An assembly, particularly the electromagnet EM, for driving a single print wire will be called a wire drive element. The wire-dot print head has a multiplicity of, e.g., 24, drive elements. They are disposed in an array or in sequence along a ring as shown in FIG. 3. In FIG. 3, the positions of the drive elements, particularly the electromagnets on the disk-shaped base yoke 10 are illustrated. They are numbered, as #1, #2, #3, and so on, in the order in which the front ends of the corresponding print wires 1 are arranged from top to bottom. The electromagnets are disposed along the circle in counterclockwise sequence, arranged in the order of #1, #3, #5, . . . #2.
For the purpose of reducing the size and cost of the wire-dot print head, the base yoke 10 to which the cores 9 are fixed, the permanent magnet 4, and the front yoke 5 and the like are formed as an integral unit, and for this reason much of the magnetic circuit of the drive element is shared. As a result, magnetic flux generated from one drive element enters a magnetic circuit of an adjacent drive element, creating a magnetic interference which brings about a variation in the magnetic circuit of the above-mentioned adjacent drive element. This magnetic interference not only increases the exciting current of the coil, but also creates considerable influences on the printing operation of the armatures, such as shifting the timing of the release of armatures. These variations in armature operation due to the magnetic interference are becoming a larger problem as the speed and the printing force of the wire-dot print head are increased.
Systems have been proposed in which the time for which currents are made to flow through the coils, i.e., the time for which the coils are energized, are varied depending on the number of the coils that are simultaneously excited, and these systems are used as an effective means for reducing the above-discussed problem. One of such systems is disclosed in Japanese Patent Kokoku Publication No. 30154/1988. According to this publication, the time for which coils are energized is varied by providing: means for detecting print data signals supplied to the respective electromagnets and the number of electromagnets that are simultaneously driven responsive to the signals, means actuated in accordance with timing signals generated every predetermined pitch of movement of wire drive element, means supplying a time signal having a length corresponding to the number of the electromagnets, and means gating the print data signals with the time signal to produce drive signals for the electromagnets.
In the conventional wire-dot printers, the control is made based solely on the number of the coils which are energized, so the printing operations of the armatures are not necessarily constant. For instance, the magnetic interference gives a significant influence on the adjacent drive elements, and the degree of interference differs much depending on whether or not adjacent electromagnets are simultaneously driven. Usually, the time for which coils are energized is varied to minimize the influence with the combination of pins giving the worst armature operations. For this reason, with respect to the combination of pins for which the time coils are energized may be short, more energy than necessary is supplied to the coils, and the heating of the coils is increased, and the printing forces are excessive.
Explanation will now be made regarding the influence on the armature operations both in a situation in which adjacent drive elements are simultaneously driven and a situation in which adjacent drive elements are not simultaneously driven. As described earlier, the magnetic interference gives the largest influence on an adjacent drive element, and the greater the separation of drive elements, the smaller is the influence. The influence appears as the variation in the inductance of the magnetic circuit of the drive element, and the phenomena vary depending on the number and positions of elements that are simultaneously driven. The phenomena also vary depending on the structure and the material of the wire-dot print head. It is therefore difficult to determine whether the magnetic interference accelerates or retards the operation of the armatures.
An object of the present invention is therefore to optimize the time for which each drive element is energized.
Another object of the invention is to optimize the energization time for each drive element taking account of the magnetic interference from an adjacent drive element.